Isn't there a small module that you can swap in the instrument cluster which allows the tach to read correctly with the 6-cylinder engine?
I'm pretty sure my brother changed this module when he swapped a 6 into his 318.
Summer: BMW 135i M-Sport
Winter: 2004 Audi A4 Ultrasport 2.7tt
Year round: 6 cylinders, two turbos, and 6 speeds.